1. Technical Field
The present invention relates to a calibration method in a shape measuring apparatus that measures a three-dimensional shape of a measurement object, and the shape measuring apparatus.
2. Related Art
Hitherto, a phase shift method has been known as a method of acquiring a three-dimensional measured value of a measurement object using a three-dimensional measuring apparatus (for example, see JP-A-2010-203867).
In the apparatus disclosed in JP-A-2010-203867, a plurality of lattice fringes having different phases are projected onto a measurement object from a projecting unit, and images of the lattice fringes are captured by an imaging unit. A three-dimensional shape of the measurement object is measured from the obtained images of the lattice fringes, using a phase shift method.
Here, in this apparatus, calibration is performed prior to an actual three-dimensional measurement. In this calibration, first, a flat plate is installed on a precision stage, and the stage is moved by a distance Δz in a z direction so as to shift the phases of the lattice fringes.
Using a coarse lattice fringe, a fine lattice fringe, and a minute lattice fringe, which have different periods, in order to calculate a z-coordinate with high accuracy, candidates for a z-coordinate of each pixel in a captured image of each of the lattice fringes are calculated using an approximate expression.
A combination where a difference between the z-coordinate candidate of the fine lattice fringe and the z-coordinate candidate of the minute lattice fringe is minimized is extracted, and the z-coordinate of the minute lattice fringe of the extracted combination is set to be a calculation value.
Incidentally, in the above-described measuring apparatus disclosed in JP-A-2010-203867, the phase of an interference fringe is shifted by moving the stage by Δz in the z direction. In this case, a complex configuration for precisely moving the stage is required, and thus the time spent on calibration is lengthened. At this time, there is also a restriction such as the need to set Δz to a value having a phase change equal to or less than 2π.
Further, using lattice fringes (fine lattice fringe and minute lattice fringe) having different periods in order to calculate the z-coordinate more accurately, a phase of each pixel is calculated with respect to the captured images of the lattice fringes, and a z-coordinate corresponding to the pixel is estimated from the phase. However, there is a need to provide a phase difference of equal to or greater than 2π in both the fine lattice fringe and the minute lattice fringe with respect to a measurement range (stage movable amount) in the z direction, and thus there is also a problem in that a period to be set is restricted.